Induction and permanent magnet synchronous electric motors operate via the production of a rotating magnetic field, which is typically formed via current flowing through coil windings mounted on a stator. The coil windings typically form a group of phase windings distributed around the stator that are coupled together. For a three phase electric motor or generator three sets of phase windings are connected together in either a star or delta configuration.
For example, a six phase electric motor or generator having six sets of phase windings connected in a star configuration is illustrated in FIG. 1, where one end of each of the windings are connected at a common point, known as a star point 100.
During operation of the electric motor or generator a different voltage phase is applied or generated across each group of phase windings. Accordingly, for an n-phase electric motor or generator an n-phase voltage is applied across the respective phase windings of the electric motor or generator.
An inverter, which includes a plurality of switches, is commonly used to operate an n-phase electric motor or generator where only a DC voltage supply is available. FIG. 2 illustrates a typical three phase inverter 200 having six switches Q1 to Q6 with associated anti-parallel diodes D1 to D6 coupled to a three phase electric motor 210. The inverter switches are typically controlled via pulse width modulation or space vector control to produce a fundamental AC output voltage of much lower frequency than the switching frequency of the respective inverter switches. The control system used for operating the inverter switches is commonly based on a microprocessor but can also be implemented by other devices, for example dedicated integrated circuits (ICs), programmable logic ICs, analog and digital signal processing circuitry, or a combination of these devices.
However, if a fault should occur in one or more of the inverter switches that results in a short circuit occurring, electric currents can circulate through the coil windings of the electric motor or generator, the failed switch and the anti-parallel diodes associated with some of the other switches. This is despite all the remaining operating inverter switches being switched off (i.e. open circuit or non-conducting).
An example of a short circuit condition is illustrated in FIG. 3, where a short circuit occurs in one of the inverter switches Q5 in a leg of a three phase inverter 300 having six switches. As illustrated in FIG. 3, even if all the remaining switches Q1-4 and Q6 are switched off, circulating currents still occur through switch Q5, D1 & D3, where D1 and D3 are anti-parallel diodes associated with switches Q1 and Q3 respectively.
As a result of the unwanted circulating currents being determined by the internally generated EMFs of the electric motor/generator 210, and being limited only by the circuit impedances of the electric motor/generator and associated inverter, the unwanted currents can be of abnormally high magnitude. Consequently, the circulating currents can result in high torques being produced by the electric motor/generator. This is particularly so in an electric motor/generator that does not have controlled excitation, for example permanent magnet machines or synchronous machines with self excitation of the rotor.
The following are examples of scenarios where inverter switch failures, which result in unwanted circulating currents, can have undesirable operational consequences.
In a drive system in which an electric motor/generator can be driven by other energy or power sources, for example an electric motor/generator in an electric vehicle or wind turbine, circulating currents can result in the overheating of the coil windings, and ultimately failure of the electric motor/generator through burn out of the coil windings.
In an electric vehicle having a single electric motor acting as a traction motor, unwanted circulating currents can result in a high braking torque being generated by the traction motor. This braking torque can cause the vehicle to decelerate rapidly without warning. In a vehicle having multiple independent drive motors, for example in-wheel motors, failure of an inverter for one wheel motor can result in a braking torque being applied to that wheel, which could result in directional deviation of the vehicle.
It is desirable to improve this situation.